Sensor circuits include sensor devices that respond to an input quantity by generating a functionally related output, usually in the form of an electrical or optical signal. A sensor is a device that detects events or changes in quantities and provides a corresponding output, generally as an electrical or optical signal. A sensor device's sensitivity is generally indicative of how much the sensors output changes when the measured quantity changes.
Sensors are used in everyday objects such as touch-sensitive elevator buttons (tactile sensor) and lamps that dim or brighten by touching the base, besides innumerable applications of which most people are never aware. In recent years, the use of sensors has extended beyond the more traditional fields of temperature, pressure or flow measurement, for example into automotive vehicle control systems comprising wheel-speed sensors.
In automotive vehicle control systems, it is known to use wheel speed sensors to obtain and utilise wheel speed information for use in controlling functions, such as, anti-lock brake system (ABS) or traction control. An ABS wheel-speed sensor system measures a speed of a vehicle's wheels and converts an analog sensor signal to a digital signal for processing by an ABS controller. The ABS controller monitors and compares speed information from all four wheels. If the signal from one wheel changes abruptly with respect to the other wheels, the ABS controller understands that the wheel is beginning to lose traction. It then takes the appropriate action by applying the brakes or performing traction control. In such automotive vehicle control systems, each wheel speed sensor has a separate channel for processing the respective wheel speed.
Referring to FIG. 1, U.S. Pat. No. 5,406,485A illustrates a known wheel speed sensor circuit 150 comprising, a wheel speed sensor 18 operably coupled to an instrumentation amplifier 72. The instrumentation amplifier 72 has two input operational amplifiers 74 and 76, wherein each of the operational amplifiers 74 and 76 has one input connected to a respective sensor output lead 28, 29, and an output operational amplifier 78, having inputs coupled to the two input operational amplifiers 74 and 76 and to a positive bias voltage Vref. The output 80 of the output operational amplifier 78 comprises an alternating sensor differential voltage imposed on the voltage Vref, which assures that the output 80 will remain in a positive range.
FIG. 1 also illustrates a voltage divider 84 between Vcc and ground with six intermediate voltage taps. Vcc and the three highest taps are coupled to each of a first pair of analog switches 86 and 88, while the three lowest taps and ground are coupled to each of a second pair of analog switches 90 and 92. Each of the analog switches 86-92 also has two control inputs 94 responsive to control bits stored in registers by a central processing unit (CPU) for selecting which of the input voltages is selected as an output. The outputs of analog switches 86 and 90 provide high and low threshold voltages to a window comparator 96. The window comparator 96 input is the common mode voltage (Vcm) from the instrumentation amplifier 72. The window comparator 96 output signifies a short condition, and is coupled via line 34 to a fault timing and latching circuit 36 and to a disable function of an ND converter 32. The output of analog switch 88 becomes a threshold voltage for a comparator 98 having the sensor high lead voltage on line 28 as its other input. Likewise, the output of analog switch 92 is a threshold voltage for a comparator 100, which has the sensor low voltage on line 29 as its input. An AND gate 102 has inputs coupled to the comparators 98 and 100 and an output to the fault timing circuit 36 for indicating an open circuit condition.
The fault timing circuit 36 has separate timeout settings, both programmable by software, for short and open conditions, and latches a fault when a respective open or short condition exceeds a time limit. The range of selectable timeout periods for a short fault is between 15 and 244 micro-seconds, and the range of timeouts for an open fault is between 4 and 500 milli-seconds. For diagnosing shorts during vehicle operation, the short timeout may be set about 100 micro-seconds.
In operation, the wheel speed sensor 18 is biased by pulling up the sensor output lead 28 to Vcc and pulling down the sensor output lead 29 to ground. If the sensor becomes open circuited, the high lead 28 will transition towards Vcc and the low sensor output lead 29 will transition towards ground and stay there. The lead voltages are monitored by the comparators 98 and 100. When the high sensor output lead 28 voltage transitions above the comparator 98 threshold and the low sensor output lead 29 voltage transitions below the comparator 100 threshold, both comparators turn ‘on’ to send an open indication to the fault timing circuit 36 via AND gate 102, and the timeout period begins. If the timeout expires, an open circuit fault is latched.
In order to detect a short to ground or a battery, the window comparator 96 compares Vcm to the high and the low thresholds. If either threshold is violated, the output of the window comparator signifies a short condition and the timer for a short condition begins. If the short timeout period expires, a short fault is latched.
Therefore, the sensor circuit 150 is operable to detect a short circuit condition for an individual wheel speed sensor 18, wherein the short circuit condition relates solely to either a short between the wheel speed sensor 18 and ground, or between the wheel speed sensor 18 and a battery. U.S. Pat. No. 5,510,707 and U.S. Pat. No. 7,532,010 also describe techniques to identify a short circuit to ground in a test mode when the vehicle is (and therefore the wheels connected to wheel speed sensors are) at rest.
FIG. 2 illustrates a schematic diagram of a short circuit problem for a wheel speed sensor circuit 200. The wheel speed sensor circuit 200 comprises current amplifiers 202 having first inputs 210 coupled to a supply voltage 212. Second inputs 216 are operably coupled to the first inputs 210 via resistive element 218, and to first terminals of switching devices 220. Second terminals of the switching device 220 are coupled to second switching devices 222, which in turn are operably coupled to the individual sensors 206, 208 via channels 214 215. In some instances, if the wheel speed is the same, with the same or a low phase difference or a low sensor current difference, the impedance seen across the sensor circuits 240, 242 are the same as the channels are matched, e.g. Z_ch1=Z_ch2 . . . . In this instance, the current passing through the short 230 is, or approaches, zero, and is unable to be detected.
For safety purposes, particularly in automotive applications, it is desirable to detect all possible faults in all possible operational conditions.